Aromatic sulfonic acids, either as such or in the form of their salts with organic or inorganic bases, are widely used. For instance, xylene sulfonates and toluene sulfonates are much used as hydrotropic agents in the detergent industry. Aromatic sulfonic acids are also important intermediates in many synthetic processes in organic chemistry, since the sulfonic acid group is relatively easily replaced by other substituent groups.
Such aromatic sulfonic acids are usually prepared by reacting the corresponding unsulfonated aromatic compound with a sulfonating agent. In the past concentrated sulfuric acid has been the principal sulfonating agent employed, but oleum (which is a solution of sulfur trioxide in concentrated sulfuric acid) and, in certain instances, sulfur trioxide itself, have also been used.
The use of sulfuric acid to effect sulfonation always results, by virtue of the reaction involved, in the production of water. This water progressively dilutes the sulfuric acid employed until sulfonation ceases, due to he concentration of the sulfuric acid becoming too low for it to effect sulfonation. The figure for the lowest concentration of sulfuric acid which will effect the sulfonation of an aromatic compound is different for different compounds, but in general the sulfonation of aromatic hydrocarbons requires a minimum concentration of sulfuric acid in the range of 78 - 90% by weight. It is apparent that, when using sulfuric acid as the sulfonating agent, it will generally be necessary to employ a large excess of sulfuric acid over that theoretically needed, in order to maintain a sufficiently high concentration of sulfuric acid for the reaction to proceed to effect a high conversion of the aromatic compound present into the corresponding sulfonic acid. When an excess of sulfuric acid is so used, the end product of the sulfonation procedure is a mixture comprising the desired aromatic sulfonic acid, any unreacted aromatic compound, by-products of the reaction and excess sulfuric acid. The separation of the main ingredients of this mixture can be achieved, but only at some cost, and with the concommitant production of substantial quantities of inorganic by-products (mainly sulfuric acid). It is clear that, if a large excess of sulfuric acid has been used in the reaction, then the amount of sulfuric acid obtained as a by-product will be substantial. This acid is difficult to dispose of, either as the acid or in the form of soluble or insoluble sulfates, particularly now when effluent requirements are becoming more stringent.
The use of oleum, instead of concentrated sulfuric acid, to some extent reduces the problem of the progressive dilution of the sulfonating agent, as the water liberated in the reaction reacts with the sulfur trioxide, which the oleum contains, to form further sulfuric acid. However, there is clearly a limit to the amount of water which can be taken up in this way by a given quantity of oleum, the precise amount depending, of course, on the percentage of sulfur trioxide contained in the oleum. Although it is possible to use less oleum than sulfuric acid, it will still normally be necessary to use an excess over that theoretically needed to react with the aromatic compound. In consequence, the problem of the subsequent separation of the desired sulfonic acid from the excess sulfuric acid at the end of the reaction and the disposal of this sulfuric acid remains.
Alternative processes which have been used for the sulfonation of aromatic compounds have involved the direct reaction of the compound with sulfur trioxide. Sulfur trioxide reacts instantaneously with aromatic compounds, and it is not necessary to use a substantial excess to effect complete sulfonation. There is, therefore, no need for the reaction product to be contaminated with substantial quantities of excess sulfonating agent. However, sulfur trioxide is very highly reactive and its reactions with aromatic compounds are extremely exothermic and difficult to control. Undesirable side reactions, particularly sulfone formation, often occur to a troublesome extent. It has, for example, been found that the addition of sulfur trioxide to benzene yields about 15 - 18% of diphenyl sulfone, and, although it is claimed that, by altering the procedure, it is possible to halve the amount of unwanted sulfone produced, the percentage obtained is still undesirably high.
In attempts to overcome these drawbacks, and to moderate and control its reactions with aromatic compounds, sulfur trioxide has been used in the presence of inert diluents. Two such procedures have been evolved. However, one of these procedures is only applicable for the sulfonation of hydrocarbons of high molecular weight, which have a very low vapour pressure at the reaction temperature, and the other procedure is expensive to implement.
The first of the two procedures involves the use of sulfur trioxide diluted with thoroughly dried air to a concentration which is generally between 1-7% v/v. This process is unsuitable for sulfonating the more volatile aromatic compounds (such as for example benzene, toluene and xylene) as the amounts of such compounds entrained in the effluent air from the process are too great, not only from the point of view of cost, but also because of the toxicity hazards and fire and explosion risks if effluent air containing substantial quantities of the aromatic compound is discharged into the atmosphere. It has not been found possible to recycle the effluent air due to its contamination with small droplets of organic matter and sulfuric acid.
Apart from the fact that the process is unsuitable for sulfonating more volatile compounds, it also has other disadvantages. Thus, the volume of air used as the diluent is large, and the drying of large volumes of air is expensive. This is particularly significant as it is not possible to recycle the air. Furthermore, the scrubbing of the effluent air prior to its release into the atmosphere is difficult, and a really satisfactory method for doing this has yet to be found.
The other process where sulfur trioxide is used in the presence of an inert diluent involves the dilution of the sulfur trioxide with an inert solvent. This process, unlike that described above, is not restricted to the sulfonation of the less volatile aromatic compounds, but is inevitably expensive since the solvent must be recovered after the reaction is completed, and some losses are inevitable. Moreover, in practice, very few solvents are satisfactory since a suitable solvent must be completely inert vis-a-vis the reactants, and preferably dissolve not only sulfur trioxide but also the aromatic compound to be sulfonated and the aromatic sulfonic acid obtained from it. Liquid sulfur dioxide has been the solvent generally used commercially hitherto. This fulfils all these requirements, but its use inevitably requires expensive pressure equipment, and refrigeration plant is necessary for its recovery. It will be apparent therefore, that the use of this process will normally only be commercially justified for the manufacture of sulfonates which sell at comparatively high prices.
One further process involving the direct reaction of sulfur trioxide with the aromatic compound has also been developed, and involves maintaining a reduced pressure or near vacuum conditions in the reaction zone, thereby increasing the inter-molecular spacing of the sulfur trioxide and hence decreasing the rate of reaction. The sulfonation by this method of alkyl benzenes having from 11 to 15 carbon atoms in the alkyl group has been described. These compounds have a comparatively low vapor pressure at the sulfonation temperature. The procedure described is, however, inapplicable to the sulfonation of aromatic compounds having a high vapour pressure at the sulfonation temperature since, when using such compounds, the quantity of vapour sucked away to the vacuum pump provided to reduce the pressure would be unacceptably high. Moreover, the process requires continuous cycling of the reaction mixture through a heat-exchanger to keep its temperature from rising to a point where side reactions would become unacceptable.